The presently disclosed embodiments are directed toward methods and systems related to extended colorant printing and more particularly to optimally halftoning image spot colors rendered with extended colorant sets using ranked ordered pairing of halftone screens with colorants based on screen quality and colorant objectionability.
Digital halftoning is an important step in printing or displaying images possessing continuous color tones. Examples of such processes are used in most printing presses, ink jet printers, binary cathode ray tube (CRT) displays, and laser xerography. It is well understood that most digital color printers operate in a binary mode, i.e., for each color separation, a corresponding colorant spot is either printed or not printed at a specified image location or pixel. Digital color halftoning controls the printing of color spots formed by combinations of colorants of a colorant set, where the spatial averaging of the printed colorant spots, such as by the human visual system, provides the illusion of the required continuous color tones, also referred to as contones.
The most common halftone technique is screening, which compares the required continuous color tone level of each pixel for each color separation with one or more predetermined threshold levels. The predetermined threshold levels are typically defined for a cell that is tiled to fill the plane of an image, thereby forming a halftone screen of threshold values. At a given pixel, if the required color tone level is darker than the threshold halftone level, a colorant spot is printed at that specified pixel. Otherwise the colorant spot is not printed. The output of the screening process is a binary pattern of multiple small “dots”, which are regularly spaced as determined by the size, shape, and tiling of the halftone cell. Conventional screening outputs can be considered as two-dimensional repeated patterns, possessing two fundamental spatial frequencies, which are completely defined by the geometry of the halftone screens.
As further refinement of color printing techniques continues, the move beyond the use of 3-color printing (using combinations of 3 colorants such as Cyan (C), Magenta (M), and Yellow (Y)) and 4-color printing (using 4 colorants such as C, M, Y and Black (K)) is rapidly expanding into printing with extended colorant sets which use 5, 6 or more colorants to increase achievable color gamut. These extra colorants are sometimes called “hi-fi”, “high fidelity”, or “extended gamut” colorants, examples of which can include orange, violet, red, green, and blue. Extended colorant sets can also be used in Phototone” printing to render images with smoother gradations, reduced texture and visual noise than is possible with using conventional colorants alone. Phototone colorants can include relatively low chroma colorants, such as light cyan and light magenta, used in addition to corresponding conventional or relatively higher chroma colorants, such as the conventional C, M, Y, K colorants. Phototone colorants can also include several levels of gray and dark yellow in these extended colorant sets.
A different halftone screen is typically used for each color separation corresponding to a respective colorant, and the complete halftoned image results from the superposition of all the halftoned color separations. However, the superposition of halftoned color separations for color printing using extended colorant sets can create interference patterns, known as moire, which can be seen in the image, thus detracting from the visual appearance of the halftoned image. Each additional halftone screen required to render an image increases the likelihood of the generation of objectionable moiré.
Significant efforts have been undertaken to reduce the undesirable effects of moiré in color halftoning including the use of different halftone screens having different screen angles for particular color separations. Stochastic screens can be used to mitigate this, however, stochastic screens can lead to a noisy or grainy appearance that is inappropriate for the high quality applications typically associated with hypochromatic colorants. Accordingly, periodic clustered shape halftone screens such as, clustered dot or clustered line screens are preferred However, as indicated above, if periodic clustered screens are not selected carefully, the screens selected for each color separation may interact with one another to create objectionable moiré patterns. While solutions to the moire issue have been found for the conventional 4 colorants, efforts to find methods for halftoning 5, 6, or more colorants are ongoing. For example, U.S. Pat. No. 5,892,891 to Dalal et al. discusses using the same screen for a hi-fi colorant and its complementary colorant (e.g., cyan and orange). Those techniques are not applicable to hypocolorants. In “Halftone-Angle Combinations for N Color Separations”, M. Coudray suggests using the same screen for a lightened colorant and a different conventional colorant (e.g., light magenta and conventional cyan). However, in at least some instances this suggested technique could lead to significant moiré and color shifts for small registration errors between color separations.
US Publication No. 20080130054 A1 to Wang, et al., also proposed a halftone configuration for moiré-free N-color printing, but extension to beyond 4-color (CMYK) requires higher frequencies than those used for the CMYK screens. As the printing industry advances to using higher frequencies (it is currently common to use frequencies ≈200 lpi) for the CMYK screens, the higher frequencies required for the additional screens will be very challenging to print reliably, since the higher frequencies can be very sensitive to marking process fluctuations.
What is needed is a periodic clustered-dot halftone screen configuration for conventional colorants and hypochromatic colorants that is stable and produces low noise prints in printing techniques such as xerography.